It is known in the pipe molding art to use a cooling plug to cool the pipe while it is in the mold tunnel. The use of cooling plugs has substantial benefits when making double wall pipe where the inner wall of the pipe slides over the exterior surface of the cooling plug.
Again, according to known practice, the inner wall of the pipe is fed as a stream of molten parison from the extruder into the mold tunnel. Often times the leading edge of the cooling plug will tend to mar the inner surface of the parison as the parison is forced onto the cooling plug. This will result in unwanted distortion in the form of abrasions and scratches, etc. to the interior surface of the inner wall of the double walled pipe.